Timing Fertilizer Applications

Timing Fertilizer Applications

Pre plant Incorporation: The best time to improve soil fertility is before planting, because nutrients can be thoroughly mixed into the soil where the plant roots will be. Phosphorus is the most important element in this category, because it moves slowly. Lime or sulfur, if needed to correct pH levels, is most effective when incorporated the season before planting. This allows time for the chemical reactions that change pH to take place. Nitrogen can also be added before planting. Inorganic forms, however, are not recommended at this time, because they leach easily, and at high levels may injure the fine roots of newly-planted ornamentals. The best sources of nitrogen in organic forms are composted materials or fish emulsion.
Roots absorb nutrients most effficiently when they are actively growing in late winter/early spring or fall. The best time to apply fertilizer is about two weeks before these periods so that the nitrogen can move into the root zone in time to be available for plant use.

Application Methods

Five methods�liquid injection, drill hole or punch bar, surface application fertilizer stakes or spikes, foliar spraying, and tree-trunk injection or implants �are discussed here. Each serves a specific role depending on the site and plant health. Regardless of the method selected, the soil should be moist at the time of fertilizing to prevent fertilizer injury to the plant. Liquid Injection. Through liquid injection into the soil, fertilizer solutions are placed in the root zone. This is an excellent method for correcting nutrient deficiencies. Injection sites should be 2 to 3 feet apart, depending on pressure, and 6 to 9 inches deep. Fertilizing deeper than 9 inches may place the fertilizer below the absorbing roots, preventing plant use. When using this method in summer or during periods of drought, water should be added to dry soil.
Drill Hole or Punch Bar. A major advantage of the drill-hole system is the opening of heavy, compacted soils, which allows air/ moisture and fertilizer to move into the soil. The drill holes should be placed in concentric circles or in a grid system around the main stem beginning 3 to 4 feet from the main stem and extending beyond the drip line. Space the holes 2 feet apart and drill them 6 to 9 inches deep. The recommended rate of fertilizer for the area should be uniformly distributed among the holes and is based on the root-zone space under the tree (and not the trunk diameter). The holes can be filled either with organic materials such as peat moss or compost, or inorganic materials such as gravel, sand, or calcined clay.
Surface Application. Applying fertilizer to the ground surface is as effective as most other methods. It is best to apply the fertilizer and then water in slowly. It is okay to place fertilizer on top of mulch in landscape beds.
Fertilizer Stakes or Spikes. Fertilizer stakes or spikes that are driven into the soil contain satisfactory fertilizer materials. Unfortunately, the spacing of spikes is such that very little fertilizer comes in contact with the root system. One or two stakes per inch of trunk diameter do not represent adequate fertilizer distribution, because lateral fertilizer movement is limited in soil.
Foliar Spraying. Spraying liquid or water-soluble fertilizer on the foliage is best for correcting deficiencies of minor elements, especially of iron and manganese. This method should not be used as a means of providing all the macronutrients required by plants. To correct chlorosis, yellowing of plants caused by nutrient deficiency, several applications during a growing season may be necessary.
Tree-Trunk Injection or Implants. The infusion of liquid or implants of fertilizer is often the best method for correcting iron and manganese problems in landscape trees. This method is especially useful in areas of adverse soil pH, high moisture, or where other means of application are not practical. The wound caused by holes made in the trunk root flare will close within a growing season.